1. Field of the Invention
This invention relates to a four-wheel steering system for a vehicle.
2. Description of the Prior Art
There has been known a four-wheel steering system in which the rear wheels are turned through an angle corresponding to the front wheel turning angle. Conventionally, the rear wheel turning angle is controlled by a proportional control on the basis of a predetermined rear wheel turning angle ratio (the ratio of the rear wheel turning angle to the front wheel turning angle) which is determined according to the vehicle speed or the like.
Generally the rear wheels are turned in the same direction as the front wheels (the same phase, the rear wheel turning angle ratio is positive) in the middle-to-high vehicle speed range in order to ensure running stability of the vehicle. However such a control is disadvantageous in that a satisfactory heading performance cannot be obtained since the rear wheel turning angle is controlled by a proportional control and accordingly the rear wheels are turned in the same phase simultaneously with initiation of turning the steering wheel.
The heading performance can be improved by detecting the yaw rate of the vehicle body and controlling the rear wheel turning angle on the basis of the detected yaw rate. For example, as disclosed in Japanese Unexamined Patent Publication No. 57(1982)-44568, when the rear wheels are turned on the basis of the following formula, the rear wheels are turned in the reverse phase (in the direction opposite to the turning direction of the front wheels, which is represented by a negative rear wheel turning angle ratio) for a moment immediately after initiation of turning of the front wheels from the neutral position (where the turning angle is zero) and then turned in the same phase in response to generation of a yaw rate, whereby both the heading performance and the running stability can be improved. Such a control is generally referred to as "phase inversion control". EQU TG.theta..sub.R =-C.sub.1 .multidot..theta..sub.F +C.sub.2 .multidot.V.multidot..PSI.'
wherein TG.theta..sub.R represents a target rear wheel turning angle, .theta..sub.F represents a front wheel turning angle, V represents a vehicle speed, .PSI.' represents a yaw rate, and C.sub.1 and C.sub.2 respectively represent positive constants.
However since the conventional phase inversion control is effected in the four-wheel steering system in which the rear wheel turning mechanism is mechanically separated from the front wheel turning mechanism and is controlled by a control means into which a signal representing the front wheel turning angle is input and at the same time, the above formula includes a term which does not depend upon the front wheel turning angle .theta..sub.F (C.sub.2 .multidot.V.multidot..PSI.'), sometimes the rear wheel turning angle .theta..sub.R cannot be zero even if the front wheel turning angle .theta..sub.F is zero. For example, when a side wind acts on the vehicle body and a yaw rate .PSI. is generated while the vehicle is running straight or when the yaw rate sensor fails, the rear wheel turning angle .theta..sub.R cannot be zero even if the front wheel turning angle .theta..sub.F is zero. If the rear wheels are turned independently of turning of the front wheels, the driver will be embarrassed. Further in the conventional phase inversion control, drift can occur if the control system fails.